A clear understanding of the nature of quantitative genetic characters requires knowing how many genes and what types of developmental effects are included in the general category "polygenic variation". Almost any character can respond to artificial selection, but very little is known about the ways in which polygenes modify the development of these traits. The principal goals of this research are to test and refine techniques for the isolation of polygenic loci with comparatively large effects, to apply these techniques to model characters in Drosophila melanogaster in order to assess the complexity of sample quantitative traits, and to analyze developmental differences between selected lines. Distinctly different segregating whole chromosomes have been identified in isofemale lines from natural populations; these chromosomes carry one or more polygenic loci causing a major difference in development of sternopleural bristle number when extracted and placed on a standard genetic background. Mapping within the second and third chromosomes has localized polygenic loci affecting L2 and L4 vein formation, and more precise localization within chromosomes is now underway. Whole chromosome substitutions and intrachromosomal mapping is also progressing in selection lines carrying modifiers of response to anterior-posterior gradients for bristles in the thorax and in selection lines carrying modifiers that enhance or suppress programmed cell death in the wing and eye. Developmental analysis includes electron microscopy of selected lines, and computer simulations are used to assess the sensitivity of polygene mapping techniques.